
A frittata is very much like a pizza in that it's so versatile. You can be fairly simple and conservative with ham and cheese or you can choose ingredients that give it a distinct flare. Mexican, Italian, Mediterranean, gourmet, cheese, or go Elvis Presley-style and throw in oddball ingredients from the fridge.
Here's the general formula:
2 people - 8" or 10" skillet - 6 eggs
4 people - 10" skillet - 8 eggs
6 people - 12" skillet - 12 eggs
A frittata is similar to an omelet. Once you start cooking the eggs, they will be done within a short time, too short to cook the fresh or frozen veggies or meat. So you start by sauteeing or steaming anything that needs longer cooking.

For this sample I'm using red peppers, onion, jalapenos, garlic, rotisserie chicken and cheddar cheese. I had frozen veggies from the market, so they were all chopped and ready to go. Cook them in olive oil a few minutes on medium heat until softened ...
If using harder veggies and/or uncooked meat, like sausage, potatoes or asparagus, you need to steam/saute them. (Of course, bacon you would just fry separately.)
To steam/saute: In skillet, drizzle olive oil, add 3 Tablespoons of water and minced garlic and salt & pepper to taste, add the harder veggies and uncooked meat (not the soft yet), put the lid on the pan and steam on medium to med-high heat until veggies are just soft and tender. Take the lid off and you will notice that when the water totally evaporates, the oil is left and the veggies start to saute.

... then throw in any softer stuff that would cook quicker. In my case it's minced garlic which would have burned with the other veggies by now. Throw in the cooked chicken another minute, just long enough to heat.
Remove from heat temporarily and lightly beat the eggs in a bowl.

Make sure the veggies/meat are distributed evenly in the skillet, put back on the burner, sprinkle cheese evenly over the veggies/meat and pour the eggs on top. Do not stir. Cook until the edges are set, about one minute.
Place in a 400F oven and heat about 10-12 minutes or until eggs are set and do not jiggle.

Remove from oven and let rest away from heat for a couple of minutes, loosen the edges with a knife, place a plate upside down over the skillet and flip the frittata onto the plate.

Above is a 6-egg frittata made in a 10" skillet, so it is a more shallow result. Use the same amount of eggs in an 8" and you'll obviously have a taller result.
Some ingredient ideas:
Hearty: Ham, Asparagus, Onions, American Cheese
Italian: Hot or Mild Italian Sausage, Potatoes, Parmesan
Mediterranean: Tomatoes, Olives, Feta
Gourmet: Roasted Red Peppers, Fennel, Wild Mushrooms, Goat Cheese
BBQ Chicken: Chicken, Corn, Beans, BBQ sauce, Smoked Cheddar
Veggie: Mushrooms, Artichokes, Pesto, Mozzarella
You get the idea.
Frittata
Compliments of JennaDish
adapted from Pam Anderson, author of How to Cook Without a Book
swirl of olive oil for sauteeing
filling ingredients
salt & pepper to taste
garlic clove, minced
4-12 eggs, depending on #servings
1/3 - 2/3 cup shredded or crumbled cheese (optional)
fresh or dried herbs to taste
2 people - 8" or 10" skillet - 6 eggs
4 people - 10" skillet - 8 eggs
6 people - 12" skillet - 12 eggs
Preheat oven to 400F. Place rack in middle or just above middle position.
If using only softer veggies (onions, peppers) and cooked meat, cook veggies in olive oil a few minutes on medium heat until softened then throw in the cooked meat to warm and blend...
If using harder veggies and uncooked meat, steam/saute. In skillet, drizzle olive oil, add 3 Tablespoons of water and minced garlic and salt & pepper to taste, add the harder veggies and uncooked meat, place lid on pan and steam on medium to med-high heat until veggies are just short or being soft and tender. Take the lid off and throw in any softer veggies and continue to saute until all ingredients are just cooked.
Remove from heat temporarily and lightly beat the eggs in a bowl.
Make sure the veggies/meat are distributed evenly in the skillet, put back on the burner, sprinkle cheese evenly over the veggies/meat and pour the eggs on top. Do not stir. Cook until the edges are set, about one minute.
Place in a 400F oven and heat about 10-12 minutes or until eggs are set and do not jiggle.
Remove from oven and let rest away from heat for a couple of minutes, loosen the edges with a knife, place a plate upside down over the skillet and flip the frittata onto the plate.
Serve alone or with something light, like salad or fruit.